I racked my first ever brew from the primary to the secondary this weekend. Besides sucking up a little too much trub (about an 1" or now settled on bottom of carboy), I think I may have left the racking tube a little too high while my beer flowed into the carboy.

So here's my question: Is it likely that I overexposed the beer to oxygen? How will I know? There also seems to only be about 4 gallons, so I am worried about headspace. I did have trouble keeping my stopper on because it wanted to push out, so I am hoping that means there is enough CO2 over the beer.

You should be fine. If you're concerned about it, you can always boil some water, let it cool and top off the secondary...of course, this will slightly dilute your wonderful brew, but it will eliminate the airspace worries and likely give you a few more beers, which is never a bad thing.

I would not top off. Even if there is not more fermentation occurring, producing CO2, which there probably will be, the act of transferring will most likely cause some of the CO2 that is already in solution to come out and purge the air from the head space. You will be just fine.